1. Field of the Invention
This invention relates to image-forming compositions and photosensitive lithographic plates using the same. More particularly, it relates to image-forming compositions for use in positive type lithographic plates which can be directly made on the basis of digital signals from a computer or the like.
2. Description of Related Art
Conventionally, there have been known image-forming materials having a photosensitive layer containing an acid generator and an acid-decomposable compound, as a positive type photosensitive layer which can be solubilized by irradiation with ultraviolet radiation. For example, U.S. Pat. No. 3,779,778 discloses a photosensitive composition containing an acid generator and a water-insoluble compound having a specific acid-decomposable group, Japanese Patent Provisional Publication No. 53-133429/'78 discloses a photosensitive composition containing an acid generator and a compound having an acetal or ketal group in the main chain, and Japanese Patent Provisional Publication No. 60-37549/'85 discloses a photosensitive composition containing an acid generator and a compound having a silyl ether group. However, all of these photosensitive compositions are sensitive to ultraviolet radiation, and are made alkali-soluble by irradiation with ultraviolet radiation to form a non-image area. Consequently, these photosensitive compositions cannot form an image by means of infrared radiation as from an inexpensive and compact semiconductor laser.
As a positive type image-forming technique in which an image can be formed by exposure to infrared radiation as from a semiconductor laser and the exposed region is made soluble in a developing solution, Japanese Patent Provisional Publication Nos. 60-175046/'85 and 7-285275/'95 disclose photosensitive compositions comprising a substance absorbing heat to produce heat (an optothermal conversion substance) and a pyrolyzable compound or alkali-soluble resin. The pyrolyzable compound used therein comprises principally a photosensitive onium salt or a quinoneazide compound. However, these compounds have low pyrolytic efficiency, so that much energy is required to decompose them thermally. Consequently, these photosensitive compositions have the disadvantage of requiring a long treating time because of low sensitivity.
Moreover, Japanese Patent Provisional Publication No. 10-268512/'98 discloses a photosensitive composition comprising an optothermal conversion substance, a dissolution inhibitor and an alkali-soluble resin. However, the dissolution inhibitor comprises a sulfonic acid ester, an aromatic carboxylic acid ester or a quinoneazide compound. Consequently, this photosensitive composition has problems in that the difference in solubility in the developing solution between exposed and unexposed regions, i.e. the stability to the developing solution (latitude of development), is poor and in that the treatable area in m2 with the developing solution is small.